1. Field of the Invention
The present invention is directed to method and apparatus for the production of solar energy conversion devices, and, more particularly, to methods and apparatus for forming high quality material films on a glass substrate for improved solar heat collection panels and for large area photovoltaic panels.
A clear need has arisen for alternate sources of energy in the United States and throughout the world as traditional energy sources are being depleted. Solar energy is one of the alternate sources being examined because of its availability and apparent lack of polluting side effects. The conversion of solar energy into an energy form which is domestically usable generally takes two forms. A first form is heat energy, wherein incident solar radiation is transmitted through a suitable transparent panel to heat a material, generally a liquid contained therein, and where the infrared radiation emitted by the heated material is reflected by the covering panel and thereby retained within the heat generating unit. A second form is electricity, wherein the incident solar radiation is directly converted at a photovoltaic junction into a direct current supply which may be interconnected with external equipment to provide either a direct current or alternating current power supply for conventional use.
It is estimated that solar energy arrives on the surface of the United States at an average rate of 4100 cal/m.sup.2 -day. Over a period of a year, a square kilometer would receive an average of 1.5.times.10.sup.12 cal. For 1977, the total estimated energy consumption by the United States for all purposes will be about 20.times.10.sup.15 kcal. Thus, about 13,000 square kilometers of continental United States land receives in one year, on the average, the equivalent of all the United States energy needs. At a 5% conversion efficiency, which is obtainable from current generation solar cells, about 267,000 square kilometers or about 5% of the land area of the 48 contiguous states could provide the equivalent of the estimated 1977 United States energy consumption.
It is clear from the above discussion that enormous land areas must be covered by solar conversion equipment in order to have any appreciable impact on the energy needs of the United States. Current research programs are concentrating on obtaining more efficient solar energy converters to provide the required large area coverage. It is axiomatic that all the research programs are concentrating on ways to provide these solar energy converters at costs which are competitive with current energy sources.
In one approach to mass producing solar energy devices, a transparent vitreous substrate, such as glass, has been utilized as the surface upon which the solar radiation is incident. A glass substrate provides not only a suitable supporting structure, but also provides a surface which is particularly suitable for the formation of films, yielding improved solar conversion efficiencies. A particularly suitable film for various solar energy conversion devices is a form of tin oxide produced according to the present invention, which is highly transparent to incident solar radiation, highly reflective to incident infrared radiation, and yet obtains a high sheet conductivity for transmitting electrical current. Prior techniques for forming suitable films of tin oxide are disclosed in U.S. Pat. Nos. 3,880,663 and 3,959,565, both by Jordon and Lampkin. The tin oxide coated glass produced in accordance with the present invention and the subject patents can be used in a device converting solar energy to heat or as a transparent electrically conductive substrate in a device converting solar energy to electricity.
If it is desired to construct a device for converting solar energy directly to electricity, photovoltaic films must be deposited on the tin oxide. In one configuration, a polycrystalline layer of cadmium sulfide (CdS) is formed on the tin oxide layer and a layer of cuprous sulfide is formed at the surface of the cadmium sulfide to provide a photovoltaic junction at the junction between the cadmium sulfide and the cuprous sulfide.
2. Description of the Prior Art
It is taught in U.S. Pat. Nos. 3,880,633 and 3,902,920 and United States Patent Application Ser. No. 767,684 filed Feb. 11, 1977, all by Jordan and Lampkin, that high quality tin oxide and cadmium sulfide films are produced when suitable film-forming compositions are sprayed on a glass substrate, where the substrate is continuously heated during the spraying operation and where the spraying operation is performed in such a manner that the surface of the glass substrate being sprayed is maintained at a substantially constant temperature and substantially free of temperature gradients. These patents and patent applications teach heating the glass substrate by floating the substrate on a molten liquid, such as tin or a molten salt, at a temperature selected to optimize the formation of the desired film. It is taught that a molten liquid provides intimate contact between the glass substrate and the heat source to insure a substantially uniform temperature along the surface of the glass substrate. It is further taught that the use of a solid heating surface, such as a hot plate, results in uneven contact and, therefore, uneven heat transfer into the glass substrate whereby substantial temperature gradients are obtained along the surface.
Prior to the patents and patent application to Jordan and Lampkin, hereinabove discussed, the prior art which relates to the production of solar cells on a glass substrate was directed to the production of only small area photovoltaic cells which could be formed by directing a spray of the selected film-forming composition over the entire surface. For example, U.S. Pat. No. 2,522,531 to Mochel teaches forming a layer of tin oxide on a glass substrate while the glass is floating on a molten metal. Mochel teaches spraying a stationary glass substrate, and the spray covers the entire substrate surface at any instant of time. U.S. Pat. No. 3,148,084 to Hill et al teaches forming a cadmium sulfide film by spraying a film-forming composition onto a glass substrate which is being heated on a hot plate during the spraying. The Hill reference also teaches spraying the entire substrate while the film is being formed.
It is important to note at this point that neither the Mochel nor the Hill patents contain any teachings relative to the importance of maintaining the sprayed surface of the glass substrate at substantially a constant temperature and substantially free of temperature gradients while the spraying is being conducted. Indeed, such a consideration is of little importance in fabricating a small area photovoltaic device in which the entire surface is being sprayed at one time. The Jordan et al references teach that such temperature gradients are of particular importance in forming large area photovoltaic panels where only a portion of the glass substrate is sprayed at any one time where such spraying can introduce detrimental temperature gradients. In the Jordan and Lampkin patents and patent application, hereinabove referenced, the temperature gradients are controlled by the spraying technique and by floating the glass substrate on a molten material to resupply heat losses.
The Jorden et al references specifically teach that a uniform surface temperature is desired during the spraying operations to produce the film qualities required to form acceptable photovoltaic cells. However, use of the molten liquid heat source presents obvious equipment design difficulties, particularly in providing for an event where solidification of the heated liquid occurs. In addition, convection of the heated liquid tends to maintain the liquid at a relatively uniform average temperature and does not readily permit localized heating of the glass substrate to particularly resupply the heat which is lost in the spraying operation. A further difficulty with using a molten liquid is that conduction transmits substantial energy to the surroundings and imposes a corresponding requirement to supply energy to the molten liquid.
Yet another important factor in forming the quality films for solar energy conversion devices is the spray application itself. Defective films may be formed where the spray is incident on a unheated area of substrate or where spray debris falls to the surface or volatilized residue redeposits. These effects are particularly pronounced during formation of the SnO.sub.x layer. Since the SnO.sub.x is the base for the entire cell, an entire panel can be ruined at the first spray booth. However, these same effects occur during CdS layer formation and can act to reduce overall cell efficiency.
The disadvantages of the prior art are overcome by the present invention, however, and improved methods and apparatus are provided for forming large photovoltaic panels where such panels may be produced on a continuous basis into photovoltaic cells and where high quality films are formed over the surface of the glass substrate with superior characteristics, particularly suitable for solar energy conversion devices, and at low costs.